This invention relates to plastic molding. In one aspect, it relates to an improved die ring for use in molding plastic articles. In another aspect, it relates to a new method for blow molding plastic articles. In yet another aspect, it relates to a new parison configuration from which plastic articles can be molded. In yet another aspect, this invention relates to a blow molding apparatus.
Blow molding is a very useful technique whereby a hollow extrusion of plasticized molding material, commonly known as a parison, is confined in a mold of the desired shape, and expanded to contact the wall of the mold by gas pressure applied inside the parison, thereby forming an object of the desired shape. When the molded object has a non-circular cross-section, as for example a square or an elliptical cross-section container, sections of maximum elongation of the parison wall become thinner than the remainder, thereby forming an object having non-uniform wall thickness. Efforts to solve the problem by extruding non-circular parisons, for example elliptical or rectangular parisons, or by extruding parisons having non-uniform wall thickness, previously failed, because of non-uniform linear rates of extrusion, causing twisting, curtaining, collapsing, and/or closing off of the parison.
It is known to use die shaping to allow extrusion of a thicker walled parison into areas which tend to "thin" because of blowing greater distances. Basically, die shaping consists of opening of the die orifice in the area which will result in a thicker walled parison being extruded into the "problem" sector of the part. Increasing the die orifice opening is normally done by machining a sector at the die face, in either the land of the die ring or the mandrel, and "running out" the shaping onto the land surface to provide a smooth resin flow path. Depending on part geometry and the desired wall thickness increase, the land shaping may be one-tenth to nine-tenths of the total land length. Shaping a large portion of the land length will normally place considerable extra material into that wall area of the parison, but will frequently cause problems of parison channeling. Shaping width is also important. Too narrow a shaping may allow material to channel through the shaped sector and result in part wall thinning at the shaping "run out". In the past, this problem has been mitigated by using wide shapings and this is the reason why wide shapings are usually preferred. The effectiveness of shaping is a fine balance between depth, land, width, shear rate and resin properties.
Localized excessive flow rates or channeling are exacerbated by faster extrusion rates and narrowing of the die orifice. For blow molding dies to form light weight parts, such as one-gallon beverage containers, very little shaping was possible in the prior art. Shaping of over 0.002 inch would frequently cause the parison to collapse or close off at its free open end due to unequal flow of resin favoring the shaped sides. For that reason, the shaping of light weight beverage containers dies is very uncommon, due to poor parison control, but will become necessary for the production of very light weight beverage containers or jugs.